Field of the Invention
The present invention relates generally to a gas turbine engine, and more specifically to an industrial gas turbine engine with a second spool having a variable inlet guide vane assembly for the low pressure turbine.
Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Variable angle vanes are used to vary the mass flow through compressor and turbine passages. Compared to fixed airfoils that have integral outer and inner end walls, variable vanes have leakage areas between the airfoil and the end walls. These leakage paths create undesirable aerodynamic losses. The larger the desired swing angle of the airfoil, the bigger the challenge to minimize these gaps. The cycle benefit for having an adjustable vane throat greatly outweighs the leakage debits.
Variable inlet guide vanes are used in both a compressor and a turbine. However, the structure for a turbine variable inlet guide vane is different than for the compressor variable inlet guide vane. In a compressor, the flow path is decreasing in height as the compressed air passing through the stages of the compressor increases in pressure. Thus, the radial or spanwise height of the trailing edge of the vane decreases in the flow direction of the compressed air. This is the opposite in a turbine where the compressed gas is increasing or expanding in the flow direction. Thus, in a turbine the spanwise height of the vane at the trailing edge is increasing in height. Thus, the leakage across the ends of the vane at the trailing edge will have greater areas due to this structure.
In addition to controlling the gaps, aerodynamic forces acting on the airfoil are considered to select the optimum rotation axis. The airfoil center of pressure is the location where the moments are zero. The rotation axis placed through the center of pressure yields no additional forces over friction to articulate the vane. This center of pressure can vary on position when the stagger angle of the airfoil is changed.